Disabled Account sending messages to me

My brother just accidentally hit the “Disable Account” button on the discord settings (Pretty dumb move but not important as of now). After this, he just went ahead and make brand new discord account as a result.

However, just earlier today, I am recieving messages from his account that he just disabled. I asked my brother if he got it back, but he said he didn't. Whoever was dming behind the account was also knew other personal information. I'm concerned of this and want to know how someone got a hold of this account even though it was disabled (accidentally) or if it was hacked somehow. 

I left the account name so if Discord staff can do something about it. Many questions arise; how is it someone got a hold of disabled account? Is it hacked? Can Discord terminate it or something? I don't know if unfriending it is enough to protect other private information.